1. Field of the Invention
The present invention relates to a method of making a glass-ceramic article having a keatite mixed crystal phase in at least a part of its interior.
2. Related Art
It is known that glasses from the Li2O—Al2O3—SiO2 system may be converted into glass-ceramic articles with high quartz mixed crystals (HQMK) and/or keatite mixed crystals (KMK) as principal crystal phase. The making of these glass-ceramics occurs in several stages. After melting and hot shaping the glass is usually cooled at temperatures in the region of the transformation temperature (Tg), in order to remove thermal stresses. After that the material is further cooled to room temperature.
The starting glass is crystallized with a second controlled temperature treatment and converted into a glass-ceramic article. This ceramicizing occurs in a multi-stage temperature process, in which crystal nuclei are produced by nuclei formation at temperature of 600 to 800° C., usually from TiO2— or ZrO2/TiO2 mixed crystals. Also SnO2 and V2O5 can participate in the nuclei formation process.
High quartz mixed crystals grow from these nuclei during heating at crystallization temperatures from about 700 to 900° C. Because of the small crystal sizes of less than 100 nm optically transparent glass-ceramics are produced, which have a high quartz mixed crystal phase. Larger crystallites and thus translucent glass-ceramics based on high quartz mixed crystals may be produced by reducing the nuclei-forming content or ingredients, dependent on the material or the temperature and time course of the process.
The high quartz mixed crystals convert further to keatite mixed crystals during further heating in a range from about 900° C. to 1250° C. The temperature and time conditions for the structural phase changes are dependent on the composition. The conversion to keatite mixed crystals is connected with crystal growth, i.e. increasing crystallite size, whereby increasing light scattering occurs, i.e. light transmission is increasingly reduced. The glass-ceramic article appears increasingly translucent because of that and is eventually opaque.
A key property of the glass-ceramics made from the Li2O—Al2O3—SiO2 system (LAS system) is the manufacturability of materials, which have a best low thermal expansion coefficient (TAK) in a range from room temperature to 700° C. of below 1.5×10−6 K−1 for materials with keatite mixed crystals as principal crystal phase in addition to the residual glass phase. Glass-ceramics, which contain high quartz mixed crystals as principal crystal phase beside the residual glass phase, are materials with TAK of less than 0.3×10−6 K−1 even in this temperature range, thus a nearly zero thermal expansion. Because of the low thermal expansion the glass-ceramics with HQMK as principal crystal phase have outstanding temperature difference strength (TUF) and temperature change resistance.
Transparent glass-ceramics with high quartz mixed crystals as the principal crystal phase find application, e.g. in fire resistant glass, chimney windows, reflectors in digital protection units (beamers) or as cooking vessels. For application as cooking surfaces a reduction of light transmission to values under 50% is desired, in order to avoid observation of the apparatus under the cooking surface (e.g. with induction cooking surfaces) and to reduce the light radiation from radiating bodies, halogen heated bodies and glass burners to the desired values. This lowering of the light transmission is achieved, e.g. by coloring transparent glass-ceramics with colored metal oxides and by glass-ceramics, which are converted to be translucent or opaque.
Glass-ceramics with high quartz mixed crystals as the predominant crystal phase are most widely used for cooking surfaces. Because of its low thermal expansion coefficient (TAK) of less than 0.3×10−6 K−1 between room temperature and 700° C. these glass-ceramics have outstanding temperature difference resistance or strength (TUF) of greater than 800° C., which satisfies all requirements for a cooking surface.
A high TUF is an indispensable property for a cooking surface. The material in the cooking zone is heated to high temperatures by powerful halogen heating bodies or radiant heating bodies. These high temperatures are desired in order to guarantee rapid cooking. Of course a temperature limiter controls the heating bodies at temperatures above about 560° C., however temperatures on the glass-ceramic cooking surface of up to about 700° C. and more can occur during improper usage, such as heating of an empty pot or when a cooking zone is only partly covered.
The small thermal conductivity of the glass-ceramic of about 1.5 W/mK guarantees that the temperatures near the cooking zones drop off rapidly and their edges remains cold. This is desirable due to safety and energy-saving considerations. The requirement that a radiatively heated or gas heated cooking surface material has a TUF of 700° C. and more results from the combination of the heated cooking zones with cold surrounding areas. Because of that high thermally induced stresses are produced under a thermal load and thus the danger of breakage of the material due to damage of the surface, e.g. by scratching, increases.
Cooking surfaces of glass-ceramic with keatite mixed crystals as the predominant crystal phase have up to now found no wide spread application, because the thermal expansion coefficient (TAK) increases when a high quartz mixed crystal glass-ceramic is converted into a keatite mixed crystal glass-ceramic. The TAK increases between 20 and 700° C. to a value of α, which is mainly above 0.5×10−6 K−1. Especially good melting and devitrification resistant compositions are available with high thermal expansion coefficients. With those compositions no sufficient TUF may be obtained for modern cooking surface systems, which have heating bodies of high power.
An opaque glass-ceramic, which has keatite mixed crystals as principal crystal phase and which is colored beige with cerium oxide, is described in U.S. Pat. No. 4,977,110. It is made by crystallization on a temperature plateau between about 1025° C. and 1175° C., starting from a nucleation temperature at about 750° C. to 850° C. The heating rate amounts to a maximum of 4 K/min. The holding time at the maximum temperature amounts to 1 hour. The resulting glass-ceramic has a very high thermal expansion coefficient (TAK) of 1.5×10−6 K−1 between 0 and 300° C.
An opaque glass-ceramic based on keatite mixed crystals, which is colored dark blue with the help of iron oxide and cobalt oxide, is described in U.S. Pat. No. 5,491,115. It is made by a method in which it is heated first to a nuclei formation temperature between 800 and 850° C. with a heating rate of 5 K/min. After that it is brought to a crystallization temperature of 900° C. also with a heating rate of 5 K/min. It is held there for a holding time of 45 to 60 min. In an additional step the temperature is increased to conversion temperature of 1150° C. with a heating rate of 5 K/min prior to cooling the glass-ceramic.
In order to increase the breakage strength of the glass-ceramic plate, up to now the ceramicizing process has been controlled and the composition of the glass selected so that a keatite mixture crystal phase is present as the predominant crystal phase in the interior of the glass-ceramic, while high quartz mixed crystals are the crystal phase in the surface layer. This is for example disclosed in U.S. Pat. Nos. 4,218,512 and 4,211,820, the WO 99/06334 A1 or also EP 1 170 264 A1. Since the thermal expansion of the high quartz mixed crystals is less than that of the keatite mixed crystals a compressive tension or stress is induced during cooling of the glass-ceramic, which counteracts the strength loss by smaller surface damage occurring during usage.
After heating to nuclei formation temperatures between 650 to 760° C. or 675 to 725° C., the crystallization is performed in a temperature range between 760 to 850° C. or 825 to 950° C. in the manufacturing method according to U.S. Pat. No. 4,211,820 and EP 1 170 264 A1. Because of that a keatite mixed crystal phase already forms during crystallization. According to U.S. Pat. No. 4,218,512 the temperature is successively brought to different levels at 593° C., 752° C. and 880° C. with holding times of 2 hours at each level. According to WO 99/06334 several temperatures cycles are repeated one after the other, but each cycle has a different maximum temperature. Also after a nuclei formation stage of about 20 min at 670° C. to 800° C. within from 15 to 30 min the temperature is increased to a maximum temperature between 1050 and 1070° C. with a holding time between 11 and 29 minutes. Proportionally shorter holding times are used with higher maximum temperatures. The maximum heating rates disclosed in this patent are less than or equal to 10 K/min.
EP 1 170 264 A1 discloses temperature difference resistance or strength (TUF) data. The glass-ceramic materials of this EP reference have TUF values >650° C., preferably >700° C. Furthermore samples made according to EP 1 170 264 A1 have impact resistance of >18 cm, average breakage height, as tested with a 200 g heavy steel ball in a ball drop test. It is given as rule that the impact resistance is achievable by a suitable tempering. The disclosed TUF relates to material constants. An increase of this TUF achievable by special ceramicizing is described.